The present invention relates to a maxicode locating method.
As is known, maxicodes (FIG. 1) are two-dimensional optical codes comprising a central target element (known as bull's-eye) by which to locate the code; and a number of dark/light (normally black/white) hexagonal-cell elements surrounding the central target element and containing the coded information associated with the code.
The central target element is defined by three concentric dark-coloured (normally black) rings of equal thickness separated by two light-coloured (normally white) annular spaces of the same thickness as the rings. The innermost ring surrounds a light-coloured (normally white) circular central space.
Known locating systems comprise an optoelectronic viewing system (normally comprising a focusing device and a television camera) for picking up a two-dimensional image (defined by a matrix comprising a number of pixels) of objects (in particular, moving objects, such as parcels traveling along a conveyor belt) to which maxicodes are applied and a processing device for locating the central space of the target element in the image and so locating the maxicode. The position of the central space of the target element in fact corresponds to the center of the maxicode.
The shape of the target element in the acquired image is affected by various factors, including:
Dimension scaling. The "apparent" dimension (measured, for example, in pixels) of the target element depends on the distance between the vision system and the maxicode. Since, in real applications, it must be possible for the maxicode to be located within a wide viewing range, the apparent dimension of the target element may vary within a fairly wide range of values.
Code ovaling. The target element of a maxicode is circular, but may be deformed, in particular, compressed along an axis into an ellipse (FIG. 2), so that the target element differs considerably from the original circular shape and may assume any one of numerous variously compressed oval shapes.
Ovaling of the target element is caused by two main factors, which may occur simultaneously:
The plane of the viewing system lens is not parallel to the plane containing the maxicode (as in the case of a parcel which is not a parallelepipedon); PA1 the traveling speed of the object is not synchronized correctly with the scanning speed of the vision system. PA1 the distance between the vision system lens and the maxicode is required, for the image to be re-scaled before searching for the target element; PA1 the traveling speed of the maxicode is required, to calibrate the image scanning speed; PA1 inclination of the objects with respect to the vision system.
To eliminate the above drawbacks, the vision system is normally subjected to restrictions, which must be established highly accurately and/or maintained within a strictly predetermined range. Such restrictions may include:
Moreover, to permit scaling, known locating devices require a maxicode of predetermined size (1 square inch).